Читать книгу Tropical Marine Ecology - Daniel M. Alongi - Страница 28

Increasing ocean heat is resulting in changes in ocean circulation. Models of ocean heat content (OHC) indicate that the poleward OHC substantially reduce (increase) in the Northern (Southern) Hemisphere, with circulation changes varying among subtropical gyres and among western and eastern boundary currents (Dias et al. 2020). In the North Atlantic Current, ocean heat transport (OHT) weakens at all depths, whereas it strengthens at the surface and weakens at mid‐depth in the subtropical gyre. The Gulf Stream has weakened but the Canary and North Equatorial Currents have increased. Changes in the North Atlantic subtropical gyre and associated OHT reduction suggest that heat moving poleward with the Gulf Stream/North Atlantic Current has reduced, and the extra heat, stored passively in the gyres, transported equatorward via eastern boundary currents, and equatorial currents. Similar changes have also been observed in the Pacific and Indian Ocean. The intensification of the equatorial currents should transport extra heat both westward and eastward via the complex system of equatorial currents and counter currents. OHT associated with the Brazil Current should intensify, contributing to the poleward OHT in the Southern Hemisphere. In the South Atlantic, a shift from equatorward OHT to poleward is predicted to result in an intensification of the Brazil Current.

Some studies have suggested that the surface warming trend in the open ocean is opposite to the trends nearshore and that the poleward expansion of the Hadley cells would increase upwelling at the poleward end and decrease it equatorward (Rykaczewski et al. 2015). Although stratification has intensified over most subtropical gyres and the global ocean, the model results of Dias et al. (2020) indicate an opposite trend in tropical regions, especially in the eastern Pacific and Atlantic basins.

Whether there is an emerging trend of global ocean circulation is not yet clear, but there is a significant increasing trend in the globally integrated, ocean kinetic energy since the early 1990s, indicating a substantial acceleration of global mean ocean circulation (Hu et al. 2020). This increase in kinetic energy is especially prominent in the global tropical ocean, reaching depths of several thousand meters, and induced by a planetary intensification of surface winds. However, regional trends are diverse; the Agulhas Current has not intensified, but shallow cells in the Pacific Ocean have accelerated in response to intensified trade winds since the early 2000s, contributing to a recent warming hiatus and leading to an increased leakage of heat and freshwater in the Indian Ocean via the Indonesian Throughflow (Hu et al. 2020). Other models project a slowdown of south Indian Ocean circulation, an important region as it modulates marine life and global climate through important oceanic connections between the Pacific, Atlantic and Southern Oceans (Stellema et al. 2019). A weakening of the Leeuwin Current and Undercurrent off the west coast of Australia is projected due to reduced onshore flow and downwelling; the reduced flow is related to changes in the alongshore pressure gradient which is consistent with the projected weakening of the Indonesian Throughflow. A strong weakening in the SW Indian Ocean of the NE and SE Madagascar Currents, Agulhas Current and flow through the Mozambique Channel is predicted, as this reduced western boundary flow is partly associated with a weaker Indonesian Throughflow (Stellema et al. 2019).